A graded index lens is a rod-like lens or a fibrous lens having such refractive index distribution in its cross section that the refractive index varies from the center toward the periphery. The refractive index is ideally expressed by the following equation:n(r)=nc(1−Ar2/2)wherein nc is a central refractive index of an optical axis of lens, A is a refractive index distribution constant, and r is a distance from the center toward the radius.
A graded index lens has characteristics capable of forming an image even when both sides are flat, so that it can be easily manufactured as a small lens typified by micro-fine lens and widely used as optical components.
A rod lens array of graded index lenses set in array can cover a great size image by superimposing erect real images (1:1) from individual lenses. The rod lens array also has such an advantage that the processing of lens ends can be done by plane polishing. Due to these advantages, graded index lenses are used as image-forming optical components in a broad range of applications, e.g., copiers, facsimiles, LED printers, liquid crystal shutter printers and multi-functional printers. Graded index lenses are also used as lenses for communication.
These graded index lenses can be manufactured by, e.g., an ion exchange process. As shown in FIG. 1, an ion exchange process is a process in which a glass body 2 containing a first cation (e.g., Li+) capable of constituting a modifying oxide is brought into contact at a high temperature with a molten salt 4 containing a second cation (e.g., Na+) capable of constituting a modifying oxide to exchange the first cation in the glass body for the second cation in the molten salt. Reference numeral 3 indicates an ion exchange tank.
The distribution of the first cation and the second cation in the glass rod can be changed by the ion exchange. By the distribution of ion concentrations in the glass rod, the refractive index of the glass rod can be changed from the center thereof toward the periphery, by which a graded index lens can be manufactured (refer to FIG. 2). Further, reference numeral 6 is a refractive index distribution curve, which indicates the refractive index at the position toward the periphery. Sign r is the distance from the center toward the radius, 0 is the center of an optical axis of lens, r0 is a radius, Nc is the refractive index of the central part of lens, and Ne is the refractive index of the peripheral part of lens.
JP-B-51-21594 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses a graded index lens excellent in chromatic aberration obtained by ion exchange of glass body containing Cs in a potassium nitrate molten salt.
JP-B-59-41934 discloses a glass composition containing Li2O and Na2O as the glass composition for graded index lens, and a method of manufacturing a graded index lens having a greater angular aperture using the composition.
JP-B-7-88234 discloses a graded index lens having an angular aperture of 13° or greater and an area rate of effective field of view of 90% or higher made from a glass composition containing Li2O and Na2O in the molar ratio of Li2O/Na2O of from 1.25 to 1.5.
Lead-free lenses are required for the consideration of environmental protection. In particular in Europe, the use of lead is inhibited by the regulations such as “instruction on used electric and electronic equipments (WEEE)” and “order on the use prohibition against harmful substances (RoHS)”.
Therefore, mother glass compositions for a graded index lens not containing lead oxide is disclosed in JP-A-2001-139341 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”), JP-A-2002-121048, JP-A-2002-211947 and JP-A-2002-284543.
In conventional lead-free graded index lenses, the amount of Li is increased not to use lead. The reason for this is that lead is a necessary component to increase the angular aperture of a lens, so that a great amount of Li, which has also the effect of widening the angular aperture, must be used in place of lead to do away with lead. Further, to obtain characteristics as lens, it is necessary to maintain the ratio of Li and Na in a specific range, so that when the amount of Li is increased it is also necessary to increase the amount of Na.
The mobility of alkali ions is influenced by glass matrix (Si, Ti, Ba, Sr, etc.), alkali concentration and the ratio of each alkali concentration. In general, when the concentration of alkali increases, the glass matrix amount decreases by that part, as a result the skeletal parts of the glass become sparse, which results in the structure that the alkali ions are susceptible to mobility, so that the alkali ion mobility increases. Accordingly, when the concentration of Li becomes high, the total alkali concentration becomes high, and the alkali ion mobility increases.
When alkali mobility is excessively great, the following problems occur.
(1) Weather Fastness is Low.
Alkali ions make stains or corrosion by reacting with counter-anions such as carbonates.
Since alkali ions easily move, stain is liable to occur even at room temperature, which results in the deterioration of quality.
(2) Strength is Low.
In forming the gradient of composition from the center toward the sides of lens by ion exchange, there arises the difference in thermal expansion coefficient between the central part and the side parts due to the gradient of the composition. As a result, distortion occurs in the lens after ion exchange processing, or a residual stress is caused in the lens, by which cracks develop, so that the strength of the lens lowers. When the temperature in ion exchange processing is increased, the viscosity of the glass lowers at the time of ion exchange, and so the occurrence of distortion or a residual stress is restrained by the structural relaxation of the glass itself. On the other hand, when the temperature increases, the moving speed of ions becomes faster and ion exchange progresses too rapid, so that it is difficult to regulate ion exchange and a high quality lens having high reproducibility cannot be manufactured. Accordingly, for manufacturing a high quality lens, it is necessary to lower the ion exchange temperature to the glass transition point or lower, however, which results in the reduction of the strength of lens. A method to reinforce the surface of lens after ion exchange process for the improvement of the strength of lens is known, but such a method takes much time and raises the costs.
(3) The Reproducibility of an Angular Aperture is Lost.
Since ions easily move, the dispersion of quality is liable to occur at the time of ion exchange process. Therefore, high quality lenses cannot be stably obtained.
In addition to the above problems derived from the increase of alkali ion mobility, conventional lead-free graded index lenses have the following drawbacks.
(4) Crystallization is Easily Generated at the Time of Spinning (A Tendency to be Devitrified).
This is attributable to the fact that a great amount of Li is used, for example, the cause is the formation of the crystals of Ba—Ti—O and the like.
(5) The Chromatic Aberration of Lens is Great.
This results from the inappropriate amount of TiO.
(6) The Efficiency of the Formation of Refractive Index Distribution is Low.
This is attributable to the fact that the efficiency of ion exchange is bad. As a result, a large quantity of Li becomes necessary for the lens formation, which results in the above problems (1) to (4) and, at the same time, the increase of the costs.
(7) Temperature Dependency of Viscosity is Great.
The temperature dependency of viscosity depends upon the composition. When the temperature dependency of viscosity is great, dispersion is liable to occur in the performance of lens. Accordingly, the problem described in the above (3) is apt to occur.